Trends in Epidemiology since 1950

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Trends in Epidemiology since 1950

Overview

Epidemiology is the branch of medicine that deals with the investigation of the causes, distribution, and control of disease in the general population, rather than at the level of individual cases. Over the past 50 years there have been significant changes in disease patterns throughout the world. Epidemiology uses statistics not only to explain present disease patterns but also to help predict how they may change in the future. The explosion in the world's population that has taken place in the last half century and the vast environmental and lifestyle changes experienced worldwide have created significant shifts in the causes of mortality as well as in morbidity, the rate of incidence of diseases. The development and more widespread use of vaccines to prevent infectious diseases and of antibiotics (antibacterial drugs) also have had an important impact on morbidity and mortality. This essay looks at these changes and their consequences for the worldwide battle against disease.

Background

One of the most important factors in epidemiology in the last 50 years has been a greater emphasis on using statistics to track disease trends and to attempt to discover their causes. For example, in the 1970s a large-scale study on cancer rates throughout the United States revealed that cancers were related to a number of environmental conditions and to lifestyle choices. The relationship of smoking to lung cancer was made more obvious by this study, as was the link between high-fat diets and colon cancer. When American cancer rates were compared with those in other parts of the world, it became even more obvious that differences in lifestyle were important. While breast cancer is relatively rare among Japanese women, it is the most common cancer among women in the United States, even Japanese-American women, which indicates that lifestyle—in this case the American high-fat diet—plays a more decisive role than does genetics in the development of many cases of this cancer.

In drug therapy the second half of the twentieth century saw the development and widespread use of many antibiotics. The first effective antibiotic, penicillin, became widely available in the late 1940s, so that by the 1950s its effect on the incidence of infectious diseases such as bacterial pneumonia, strep infections, syphilis, and gonorrhea became obvious. These drugs reduced both morbidity and mortality and made a significant contribution to rising population rates in developing nations, where infection was a more significant cause of death than in developed countries. In the latter, where there was a longer life expectancy, most people lived long enough to develop degenerative diseases such as atherosclerosis, the build-up of fatty deposits in the walls of the arteries that can lead to heart attack and stroke. Most types of cancer, though they may occur in younger people, also become more common with age.

The World Health Organization (WHO) was created in 1948 by the United Nations. In its mission to improve the health of people everywhere, especially in developing nations where healthcare was usually scarce, WHO set out to attempt to find cost-effective ways to reduce morbidity and mortality while at the same time developing better statistics on disease incidence worldwide. One of WHO's most successful projects was the elimination of smallpox , an often fatal infection that left the skin of survivors seriously scarred from the pustules that covered the body. WHO mounted a large-scale international vaccination program coupled with aggressive treatment and isolation of those with the infection. The last smallpox case was treated and cured in 1977, making smallpox the first known infectious disease to be completely eradicated.

The elimination of smallpox led to the hope that other infectious diseases could also be eradicated, but at the end of the twentieth century smallpox remains the only such success. In economically advanced countries polio was brought under control in the 1950s and 1960s with the vaccines developed by Jonas Salk (1914-1995) and Albert Sabin (1906-1993), but the battle against polio is still being waged in developing countries. Malaria remains one of biggest health problems worldwide despite a period in the 1950s during which its incidence declined rapidly. Malaria is caused by a one-celled parasite transmitted by mosquitoes, and in the 1950s the pesticide DDT was used extensively to eliminate mosquito populations. This led to a significant drop in the incidence of malaria, but within a few years mosquitoes became resistant to DDT and their populations rose again, and the scourge of malaria returned in developing nations in the tropics. Though anti-malarial drugs were developed, the parasite eventually became resistance to them, rendering them ineffective. In the 1990s, substantial progress was made on a vaccine, but malaria remains a particularly difficult health problem. In many tropical areas it is endemic, meaning that it is likely the people there will contract the disease, usually early in life. Since the parasite remains in the body, it periodically continues to damage the red blood cells, sapping its victims' energy and lowering resistance to other infections.

Impact

The history of malaria points up the relationship between the environment and disease, a relationship that can often be extremely complex. For example, it might seem that economic improvement projects that bring increased employment and other benefits to underdeveloped areas would also lead to improved health. But in fact the incidence of schistosomiasis, which is caused by a snail-borne parasite, has increased in areas of such projects because snails breed in drainage and irrigation canals. There is also evidence that the Ebola virus—a dreaded virus that causes rapid illness and death in most of its victims—becomes more likely to spread from apes to humans as human populations replace forested areas with farmland, bringing the apes into more direct contact with humans.

Human immunodeficiency virus (HIV) may also have spread from an isolated indigenous population in Africa that had developed a resistance to it. Individuals in this group may have had more contact with people from other areas, as African nations in the 1950s and 1960s became more urbanized and as their transportation systems improved. HIV is an example of how, despite the conquering of some infectious diseases with vaccines and antibiotics, humans still remain vulnerable to infections. For example, acquired immune deficiency syndrome (AIDS) was first identified by U.S. physicians working with male homosexual patients in the early 1980s. Soon it was also discovered among U.S. intravenous drug users and those who had received tainted transfusions and blood products. It was found to be a global rather than a national problem as cases were identified all over the world, but particularly in Africa, making it apparent that this continent was the likely source of the infection. By 1983 HIV was identified as the cause of AIDS, and by the mid-1990s relatively effective drugs were developed that slow the virus's destruction of the immune system. But these drugs are expensive and their effective use requires the kind of healthcare systems that only exist in developed nations. In developing nations such as those in Africa, where 69% of the people with HIV or AIDS live and where 84% of the deaths from the disease in 1999 occurred, such drugs and healthcare are completely lacking. It is estimated that by 2010, life expectancy in Africa will drop from 59 years to 45 because of AIDS. The incidence of HIV has also dramatically increased in the 1990s in Asia and Latin America.

While emerging infectious diseases such as HIV pose a critical health problem in the world today, many more common infections that have long been on the scene continue to have significant impact. Particularly in developing nations, diarrheal infections such as dysentery still claim many lives, particularly among infants and young children. These infections are often water-borne and are common in areas where a clean water supply and sewer systems are nonexistent. Respiratory infections also kill many children in developing countries, in part because antibiotics are not as readily available as they are in the developed world. While these problems have long plagued these areas, new threats are emerging with increased urbanization and industrialization. These often bring increased air and water pollution as well as exposure to many toxic industrial wastes.

In the developed world the patterns of disease are quite different, but nonetheless disturbing. While over the past 50 years the death rate due to heart disease has decreased by more than a third in the United States, the death rates for most cancers have remained relatively steady and in some cases have actually increased. While the decrease in the incidence of and death from heart disease is very clear, it is still difficult to pinpoint its causes. Most experts attribute it to a variety of factors involving both lifestyle changes and improved healthcare. For instance, fewer Americans smoke today than they did 50 years ago. Also, more people are weight conscious and are aware of the importance of a low-fat diet and exercise to a healthy heart. Fifty years ago, there was little that could be done to prevent or treat heart disease, but now there are cholesterol-lowering drugs and a wide variety of blood-pressure reducing medications, both of which combat the risk factors for heart disease. There are also a number of medical techniques that were developed, including bypass surgery, to counteract the effect of clogged arteries that lead to heart damage.

With cancer there have also been definite improvements in treatment and prevention over the past 50 years. Childhood leukemias that were almost always fatal 50 years ago now have a survival rate of more than 50%, a change largely due to the development of effective chemotherapy. The low-fat diet that helps to prevent heart disease will also lower one's risk of colon and breast cancer. Yet statistics indicate that one out of nine American women will develop breast cancer. Lung cancer is also becoming more common among women, since less women than men are giving up cigarettes and more young women than men are taking up smoking. In other parts of the world smoking-related illnesses are also on the rise, since the anti-smoking campaigns that have been effective in the United States are quite uncommon elsewhere.

This brief survey of the world health trends over the past 50 years indicates how important epidemiology is to our understanding of disease patterns and how they are changing. In the next 50 years disease patterns are likely to continue to change as new treatments as well as new diseases emerge. The median age of the population in developed countries will continue to increase, which means that degenerative diseases will become an even bigger problem, while in most developing nations the population will continue to increase significantly despite the development and increased use of contraceptives. This means that in these nations the median age will continue to remain low and that infant mortality, problems of malnutrition, and poor sanitation are likely to remain crucial factors in causing disease.

ROBERT HENDRICK